Mortalités estivales de l'huître Crassostrea gigas dans le bassin d'Arcachon : facteurs du milieu, aspects biochimiques et histologiques

Previous pathological studies have not permitted to implicate infectious disease in summer mortalities of oysters Crassostrea gigas, in the Arcachon bay. In 1983, these summer mortalities, occurring mostly among 1 year old oysters, have been the subject of new investigations. In order to establish the causes of mortalities, oysters of the same origin were held in two areas characterized by different prevalence of mortality for 6 months, during which measurements of physiological and biochemical condition and certain histological observations were made. The study has been completed by environmental observations. Different rates of mortality appeared between the two selected areas: 30 %in Taussat in the upper part of the bay, under a continental influence and 3 % in Cap Ferret, under an oceanic influence. Among environmental factors, temperature showed the most significant difference between the two sites, with the highest values of 27°C in Taussat and 22°C in Cap Ferret. In the two areas, the nutrient levels appeared to be similar and no deoxygenation was ever observed during the summer. Statistical size difference between surviving and dead oysters was not observed, and the sex ratio data indicated that the mortality was not selective against females or males. Mortality occurred after gradual decrease of the relative dry weight and the condition index of the oysters, and was more marked in Taussat. In both groups studied, the timing of mortality also coincided with the decline in carbohydrate content from 9 % of the dry weight to values as low as 0,7 %. The histological studies confirmed the absence of pathogens, and showed a change in the digestive cell structure: a thinning of the digestive tubule epithelium resulting from a loss of the apical cytoplasm in response to exposure at elevated temperature. This epithelial thinning appeared to be related to the formation of autolysosomes in the digestive cells. This change was greater in Taussat oysters than in those of Cap Ferret. These data suggest that hi.gh temperature, sexua l maturation, and spawning constitue various stresses increasing the physiological needs of oysters. The animals show an energy imbalance, resulting in the utilization of fat reserves when carbohydrates are depleted, which may be related to the process of autophagy described above. In severe cases, it can represent a pathological condition. [NOT CONTROLLED OCR] Proceedings of the first international colloquium "Pathology in marine aquaculture" held from 11 to 14 September 1984 in Montpellier, France. European Aquaculture Society. Dimensions: 16 x 23.5 cm, XVII + 428 p. ISBN-774-0689.

En l'absence d'éléments permettant d'avancer une étiologie infectieuse des mortalités estivales de Crassostrea gigas dans le bassin d'Arcachon, une étude comparative d'un site à haut risque de mortalité (Taussat) et d'un site à bas risque (Cap Ferret), a été entreprise au cours de l'été 1983 sur des Huîtres de 1 an. 30 % de mortalités affectaient des lots expérimentaux à Taussat et 3 % au Cap Ferret. Hormis des différences de température, maxima: 27°C à Taussat et 22°C au Cap Ferret, aucune désoxygénation du milieu n'a été mise en évidence. Sur le plan nutritif les deux sites sont similaires. Les mortalités s'observent à la suite d'une décroissance progressive des poids secs relatifs et des index de condition des Mollusques et après épuisement des réserves glucidiques (0,7 % du poids sec). Les examens histologiques ont révélés une altération de l'épithélium digestif consécutive à une activité autophagique caractéristique d'un état de stress. Il est suggéré que l'augmentation des besoins énergétiques des Huîtres due à des fortes températures à la maturation et à la ponte, au moment où les réserves glycogénées sont au plus bas, entraîne, un déséquilibre métabolique qui peut dans ces cas aigus, représenter un état pathologique. [OCR NON CONTRÔLE] Actes du 1er colloque international "Pathologie en aquaculture marine" (11-14 Septembre 1984), Montpellier, France. European Aquaculture Society. Dimensions: 16 x 23.5 cm, XVII + 428 p. ISBN-774-0689.